Modern combat aircraft are provided with a sophisticated Inertial Navigation System (INS) which determines the aircraft's attitude within a geographical coordinate system. These aircraft may also be provided with various types of sensing systems (known generally as avionics) which are often mounted at locations remote from the INS. One important class of these avionic systems are target locating systems. These systems locate hostile targets and calculate a vector from the avionic mounting location to the target. A weapons control system then uses the information from the target locating avionics and aircraft attitude information from the INS to aim and fire aircraft-mounted weapons at the target. These precision target locators, aircraft attitude sensors, and weapons guidance systems work together to create extremely accurate weapons delivery systems.
Unfortunately, the accuracy achieved by these guidance systems can be significantly impaired as a result of mechanical flexures induced in the frame of the aircraft during flight. Such flexure may come about, for example, by the application of different aerodynamic loads on various surfaces of the aircraft during flight, or different weight distributions within the aircraft as fuel is exhausted, etc. Because this flexure in the frame of the aircraft distorts the relative positions of the INS and the remotely-located target identification avionics, it can cause errors in weapons aiming. As the precision of the sensing and guidance systems have been improved, they have become so accurate that normal airframe flexure, even though it may produce only a thousandth of a radian of error in weapon-to-target vector calculations, is a primary limiting factor in overall system accuracy. This problem has in the past been overcome either by mounting all avionics in close proximity to the INS, or by providing the avionics with a dedicated attitude sensor of INS quality. These solutions have not been entirely satisfactory. It is not always practical, given weight distribution, space allocation, and avionics performance considerations, to mount avionics next to the INS. The provision of a second INS at the avionics location is prohibitively expensive and cumbersome, as an INS quality attitude sensor costs upwards of $300,000 and weighs about 50 pounds. While the use of a low-cost attitude sensor at the avionics location has been considered as a possible solution, the long term drift errors and other inaccuracies associated with the outputs of such low cost units has thus far prevented their use in this context.
Therefore, there is a need for a cost-effective system which accurately and reliably compensates for airframe flexure between the INS and avionics equipment.